Over the next couple of decades, NASA wants to extend the human presence into the Solar System, including onto the surface of Mars. Elon Musk founded SpaceX expressly to colonize the Red Planet by the 2020s or 2030s. For now, both the government and private sector have focused on “hardware”: the rockets, spacecraft, and technology needed to keep humans alive en route to Mars and on its surface. And these are not simple or inexpensive problems to solve.

One could argue, however, that the “software” problems may prove even more challenging. That is, can an embryo mature in space, be born, and grow into a healthy human being?

After a 12-day experiment in space this month, Chinese scientists claim they have done so. On April 6, the SJ-10 spacecraft launched from the Jiuquan Satellite Launch Center in Gansu province carrying about 20 different experiments in a retrievable satellite module. It returned to Earth on Monday, landing in inner Mongolia.

One of those experiments was housed in a microwave-sized chamber with a cell culture system that contained 6,000 mouse embryos. During a 96-hour experiment, some of the embryos developed into advanced blastocysts. Photos of these developing embryos were transmitted back to Earth via a satellite and later published by Chinese media.

A Stanford University professor of obstetrics and gynaecology familiar with the research, Aaron Hsueh, said determining the exact percentage that developed must await further analysis of fixed samples from inside the capsule. Huseh, who researches reproductive health, nevertheless told Ars that this experiment represents a significant advance in the field of space reproduction. "Yes," he said. "It shows that microgravity does not deter mammalian embryo development."

Some US scientists said the new results, while promising, should be approached with caution. A senior researcher in the field of reproduction, Joseph Tash at the University of Kansas, told Ars that it will be important to know what proportion of the thousands of embryos did develop and what proportion were healthy and normal. "From the image that was distributed, there appears to be a non-uniform population of embryos in different stages of quality and development," he said. "The use of the word 'some' developed is troublesome without knowing more details of the data and the statistical analysis. If only a small percentage developed normally compared to the ground controls, this raises concerns about the nature of the space effect on embryo development. I would reserve judgment until the data and methods become available for independent scientific scrutiny."

To colonize space, we're going to have to be able to have sex and make babies there. It seems likely that humans have at least done the deed in orbit. If nothing else, the crew of STS-47, which flew in 1992, included the married couple N. Jan Davis and Mark C. Lee. They declined to discuss any sexual activity after the flight. It's also clear that female astronauts returning from space have gotten pregnant within a few weeks or months of their space shuttle missions.

But that’s about it when it comes to humans and sex research in space—at least officially. Between Russia, the United States, and China, there has not been all that much work done even with non-human mammals. And what has been done until now has not favored easy reproduction in orbit.

Rather than in space, most reproductive studies have been performed on Earth in rotary cell culture systems, which mimic the microgravity of orbital spaceflight. A 2009 study found that simulated microgravity disrupted early embryo development in mice. Another study of mouse egg cells found that their rate of maturation fell significantly in the absence of gravity, from 73 percent in Earth gravity to nine percent in simulated microgravity. And in yet another Earth-based study, researchers found that simulated microgravity alters expression of genes related to heart formation. There are many others, most of which point to potential trouble spots in human reproduction in non-Earth gravity.

NASA has done some work with mammalian embryos in space. During the STS-80 space shuttle mission in 1996, two-cell mouse embryos were launched into space and cultured for four days in microgravity. They showed no signs of development, whereas control embryos on Earth developed to normal blastocysts. Later, six mice were flown aboard space shuttle Discovery to the International Space Station, and of those, three returned alive after a 91-day mission. Either due to radiation, microgravity, or both, researchers found that sperm counts were decreased by 90 percent in these mousetronauts. NASA is also currently conducting "Space Pup," experiments aimed at gaining a better understanding of how radiation affects cellular development. In this experiment, freeze-dried mouse sperm are flown aboard the International Space Station for up to 24 months and then fertilized on Earth.

Julie Robinson, the program scientist for the International Space Station, said earlier experiences with mammals in space have led NASA to be more conservative with reproductive studies. "Most scientists would say we are still learning the basics of keeping mammals healthy enough in space, and so we would not come back to reproductive studies until we had solved a number of different issues—group housing, nesting, nursing, stress responses, and general animal care," she said. "Rodent reproductive studies are listed in the Decadal Survey on Life and Physical Sciences put out by the National Academies, but it was acknowledged we aren’t ready scientifically at this point to carry them out."

Russia, too, has done some innovative experiments. Back in 1979, the Soviet Union flew the Cosmos 1129 mission into space during which it hoped randy male and female rats would get it on in microgravity inside a "common breeding chamber." Although some ovulation occurred, no females gave birth.

China has tried mammal embryo experiments before. Prior to the SJ-10 mission, in 2006, the Chinese space agency launched the SJ-8 orbital mission, which carried four-cell embryos into space, but none of them showed signs of development. Scientists weren't sure whether this was due to the lack of gravity, higher gravity during launch, or some other factor.

A general review of some of these studies and overall human reproductive health in space published in 2014 concluded, "Despite the consideration that in the distant future, human reproduction is likely to occur in space, the current literature base is insufficient, limiting speculation about the possibility that intricate and complex phases of reproduction in mammals—including mating, fertilization, implantation, placentation, embryogenesis, organogenesis, prenatal and postnatal development, birth, lactation, and suckling—can occur in space."

So what might all of this mean for the colonization of Mars? Nearly all of this research has occurred in microgravity. On Mars, which has one-third the gravity of Earth, some of the problems with human reproduction due to zero gravity would presumably be less pronounced.

However, the radiation environment on the surface of Mars is far more harsh than even aboard the International Space Station. All of which is to say, if we know almost nothing about human reproduction in Earth orbit, we know less about sex and babies on Mars.

Scott Solomon, a biologist at Rice University in Houston, has given a lot of thought to these questions, and he believes it may be biology, rather than physics, that proves the true stumbling block for human expansion into the Solar System. Asked for his advice to NASA, Musk and others who would see humans living on the Red Planet, Solomon offered the following, "We need to learn a lot more about what is involved in human reproduction off Earth. For us to have a long-term colony, it’s going to necessarily involve people having babies. We should do more tests on other species. We have had some mice tests in low-Earth orbit, but not as much as you might expect if this is a serious goal for people to actually be doing this within the next few generations. People should probably be thinking more carefully about this."

Now that China appears to have had some success with the first step, embryo development, perhaps they will.